Fitness tracking devices, smart watches, and other wireless health monitoring devices have been found to be capable of sensing, recording, and transmitting a number of health-related information. These devices are now helping to track users' walking, running, and the like; to identify and track heart rate; and to encourage patients to remain active regularly throughout the day; often through the use of a relatively small, unobtrusive, wrist-worn device.
Along with monitoring of standard activity parameters, it may also be desirable to monitor bioelectrical signals (e.g., EKG, body impedance) in an easy-to-use ambulatory setting. Unfortunately, measurement of the desired relatively small signals from the heart often involve coupling expensive, temporary electrodes to a chest of a patient using gels and/or adhesives. While selected electrical health measurements can be obtained through skin of an individual's limbs using research or clinical measurement systems, the small amplitude of many of the signals of interest, the noise generated in the body and at the interface between the measurement system and body, the distance between (for example) the heart and the wrist, and the like have limited the monitoring of electrical signals from the body outside of the clinic and/or in the absence of large and expensive measurement systems.
Despite the limitations of existing technologies, impedance measurements may be used in a variety of different approaches for assessing the health of a subject. For example, bioelectrical impedance analysis (BIA) is a non-invasive technique that can be used to measure body composition in terms of percentage body fat. As another example, impedance cardiography (ICG) can use electrical and impedance signals to detect the properties of the blood flow in the thorax of a subject. The electrical and impedance signals can be processed to measure and calculate various hemodynamic parameters, such as heart rate, cardiac output, the amount of blood pumped by the left ventricle each heartbeat, the resistance to the flow of blood in the vasculature, peak acceleration of blood flow in the aorta, peak velocity of blood flow in the aorta, thoracic fluid content, the pre-ejection period (the time interval from the beginning of electrical stimulation of the left ventricle to the opening of the aortic valve), and left ventricle ejection time. As another example, Electrical Impedance Myography (EIM) is a non-invasive technique that can use the electrical impedance of individual muscles as a diagnostic tool for a number of neuromuscular diseases. EIM measures changes in muscle composition that occur during disease progression.
Impedance measurements can be made using a tetra-polar measurement method. Tetra-polar measurement methods may be more accurate than bi-polar measurement methods, but may suffer from parasitic impedances or coupling as the form factor of the measurement device gets smaller.